[Runners] Use icon name
Summary: QIcon isn't thread-safe and here we just pass a theme icon name anyway, so we can have the runner UI do the work as needed.
Test Plan: Compiles
Reviewers: KDE Connect, nicolasfella
Reviewed By: KDE Connect, nicolasfella
Subscribers: kdeconnect
Tags: KDE Connect
Differential Revision: https://phabricator.kde.org/D19158